onside kick question

[ase]

i am more confuse now. sorry guys

what is the difference between the following two situation

(1) normal kickoff (kicking team just score TD or FG) or free kick, kicker kick it in the air and ball hit the ground on receving side's 15 yard, kicking team down the ball. Result: receving team's possession

(2) normal kickoff (kicking team just score TD or FG) or free kick, kicker kick it short for 15 yards, hit the ground, kicking team down the ball. Result: kicking team's possession

the result are different, but they all kick more than 10 yards and all hit the ground, so what i really want to ask is this in what situation do you apply rule 2 and call it onside kick, and in what situation do you apply rule 1?

thanks again, sorry for being so dumb